I've only worked on W116 wipers. It could be that one of the ball and socket joints in the wiper assembly came apart. You might be able to just pop it back together. If the W126 is anything like the W116, it should quite easy to get to the wiper assembly. I hope it's not like the W116 that needs new windshield grille rivets.
If you have to take the assembly apart, consider new seals where the wiper shafts go through the case. A bad seal led to corrosion inside the wiper assembly in my W116.
78 450SEL (sold)